Primary Invoice Format

This is a required field when setting up a project's invoice attributes.  The list of available options is defined by the Administrator on the Admin >> Setup >> Invoice Formats screen.  

Changing the Invoice Format on a project will have no impact to any existing draft or completed invoices and will only affect the layout of future invoices.

Note that this is designated the Primary Invoice Format.  Using the Additional Invoice Formats tab, you can optionally specify additional Invoice Formats to be generated along with the Primary.